Problem: The scale on a map is 6cm : 8km. If two cities are 42cm apart on the map, what is the actual distance in km between the two cities?
Answer: The scale means that every 6cm on the map corresponds to an actual distance of 8km. A distance of 42cm on the map is the same as 7 $\cdot$ 6cm. The actual distance between the two cities is 7 $\cdot$ 8km, or 56km.